1. Field of the Invention
The present invention relates to a self-oscillation switching power supply apparatus.
2. Description of the Related Art
A ringing choke converter is widely used as a self-oscillation switching power supply apparatus. FIG. 9 is a circuit diagram of a ringing choke converter (hereinafter referred to as an RCC) according to a conventional technique. As shown in FIG. 9, a switching transistor Q1 is connected in series to a primary winding N1 of a transformer T. A control circuit including a phototransistor PT serving as a photosensing element of a photocoupler is connected to a feedback winding N.sub.B of the transformer. A control transistor Q2 is connected between the gate and the source of the switching transistor Q1.
A rectifying and smoothing circuit including a rectifying diode D3 and a smoothing capacitor C5 is disposed between two terminals of a secondary winding N2 of the transformer T. The output of this rectifying and smoothing circuit is connected to a voltage detecting circuit including a resistance voltage divider consisting of resistors R9 and R10, a shunt regulator SR, a light emitting diode PD of the photocoupler PC, and a resistor R8.
The circuit shown in FIG. 9 operates as follows. When the circuit is started by turning on a power supply, a voltage is applied to the gate of the switching transistor Q1 via the starting resistor R1 and the switching transistor Q1 turns on. As a result, an input power supply voltage is applied across the primary winding N1 of the transformer T and a voltage with the same polarity as that of the primary winding N1 is generated across the feedback winding N.sub.B. This voltage signal is applied as a positive feedback signal to the gate of the switching transistor Q1 via a capacitor C2 and a resistor R2. Furthermore, the voltage induced across the feedback winding N.sub.B causes a charging current to flow into a capacitor C3 via a diode D1, resistors R3 and R5, and the phototransistor PT of the photocoupler. If the voltage across the capacitor C3 exceeds the forward base-emitter voltage of the control transistor Q2, the control transistor Q2 turns on. As a result, the gate-source voltage of the switching transistor Q1 becomes nearly 0 and thus the switching transistor Q1 is forced to turn off. As a result, a voltage is generated across the secondary winding of the transformer. This causes the rectifying diode D3 to have a voltage applied in the forward direction. As a result, the energy which has been stored in the transformer T during the on-period of Q1 is released via the secondary winding N2 and the capacitor C3 is reversely charged by a flyback voltage of the feedback winding N.sub.B via resistors R6 and R7 and a diode D2.
If the voltage across the capacitor C3 becomes lower than the forward base-emitter voltage of the control transistor Q2, the control transistor Q2 turns off and the energy stored in the transformer T is released from the secondary winding. If the current passing through the rectifying diode D3 becomes 0, a kick voltage is induced across the feedback winding N.sub.B whereby the switching transistor Q1 again turns on. After that, the above process is repeated.
In the above operation, the output voltage across the load is detected by means of a resistance divider comprising resistors R9 and R10 and the detected voltage is applied as a control voltage to the shunt regulator SR. According to the detected voltage, the shunt regulator SR changes the current passing through the light emitting diode PD of the photocoupler. As a result, a corresponding change occurs in the amount of light received by the phototransistor PT serving as the photosensing element of the photocoupler, and thus the impedance of the phototransistor PT changes. This causes a corresponding change in the charging time constant associated with the capacitor C3. Because the charging time constant increases with the reduction in the output voltage, a reduction in the output voltage results in an increase in the period of time from an off-to-on transition of the switching transistor Q1 to the following on-to-off transition forcedly brought about by the control transistor Q2, that is, an increase in the on-time of the switching transistor Q1, which results in an increase in the output voltage. As a result, the output voltage is controlled at a constant value.
It is known that the oscillation frequency f of the switching transistor Q1 in the conventional self-oscillation switching power supply apparatus such as that shown in FIG. 9 varies in approximately inverse proportion to the input or output power as shown in FIG. 10 in which the oscillation frequency f is plotted as a function of the output power Po.
In general, the switching loss which occurs during each switching operation decreases with the reduction in the load. However, since the oscillation frequency f increases, as shown in FIG. 10, with the reduction in the output power Po and thus with the reduction in the load, the frequency of occurrence of switching loss per unit time increases with the increase in the oscillation frequency f. Therefore, the reduction in the switching loss which occurs when the load decreases is very small. This means that the efficiency of the power supply apparatus decreases with the reduction in the load.
The switching loss under low load condition can be reduced by designing the circuit parameters such that the oscillation frequency for the operation under the rated-load condition becomes low enough. However, in the case where the power supply apparatus is required to handle a load varying over a wide range from extremely low to high levels, it is necessary to set the oscillation frequency f under the low load condition to a relatively high value. That is, the oscillation frequency under the rated-load condition is generally determined by factors associated with components such as the magnetic flux density of the transformer and other factors such as ripples and noise. If the oscillation frequency is set to a too low value, problems such as saturation of the transformer occur.
In view of the foregoing, there is a need for a self-oscillation switching power supply apparatus capable of operating without a reduction in the efficiency due to an increase in the oscillation frequency under a low load condition even in the case where the output power to load varies over a relatively large range.